Engine emissions compliance requires the detection of air-fuel ratio (AFR) imbalances across all engine cylinders. An AFR imbalance between cylinders may occur when the AFR in one or more cylinders is different from the other cylinders due to issues such as intake manifold leakage, fuel injector errors, exhaust gas recirculation errors, and fuel flow delivery issues. In addition to degrading emissions, cylinder-to-cylinder imbalances can result in torque errors that reduce engine performance and vehicle driveability.
One example approach for detecting cylinder-to-cylinder AFR imbalances is shown by Behr et al. in U.S. Pat. No. 7,802,563. Therein, AFR imbalance is identified based on the response of an exhaust gas UEGO sensor at frequencies that are at or above a firing frequency of the cylinders during selected operating conditions. Specifically, when the vehicle is not in transient engine operating condition, imbalance is identified if the integration of high frequency differential signals detected by the UEGO sensor is higher than a threshold. Still other approaches for AFR imbalance detection involve detecting AFR imbalance based on exhaust manifold pressure. However, the inventors herein have recognized potential issues with such methods. As one example, when using exhaust gas sensors, as in the approach of Behr, there may be conditions where cylinder-to-cylinder imbalance is not detected due to insufficient mixing of exhaust gas at the exhaust gas sensor. Further, the exhaust gas sensor may not be able to reliably detect cylinder-to-cylinder imbalance during an engine cold-start condition due to insufficient warm-up of the exhaust gas sensor. As another example, when using exhaust manifold pressure to detect AFR imbalance, the detection may be affected by the distance between the pressure sensor and the cylinder. With increased distance, exhaust gas from other cylinders is more likely to mix with the exhaust gas from the cylinder under estimation. In other words, the reliability of any given approach may vary based on operating conditions. As such, if a cylinder fuel or air injection is adjusted responsive to an indication of AFR imbalance during conditions when the sensor output is not reliable, further AFR and torque issues may be generated.
In one example, the issues described above may be at least partly addressed by a method comprising: indicating cylinder-to-cylinder imbalance based on each of exhaust air-fuel ratio estimated by an exhaust gas sensor, exhaust manifold pressure estimated by a pressure sensor, and individual cylinder torque estimated by a crankshaft torque sensor. In this way, cylinder-to-cylinder imbalance may be more reliably identified over a broader range of engine operating conditions over a given drive cycle.
As one example, each of exhaust AFR, exhaust manifold pressure, and individual cylinder torque may be estimated at different operating conditions over a given drive cycle. Cylinder-to-cylinder imbalance may be identified by weighting each of the estimated exhaust AFR, the estimated exhaust manifold pressure, and the estimated individual cylinder torque with a confidence factor. The confidence factor may be adjusted based on the type of estimation and the operating condition at which the type of estimation was performed. For example, the confidence factor of an imbalance estimation based on the output of an exhaust gas sensor may be decreased during conditions when exhaust mixing is lower, and increased during conditions when exhaust mixing is higher. The confidence factor of an AFR estimation based on the output of a pressure sensor may be decreased as the distance between the pressure sensor and the exhaust valve of the cylinder is greater than a threshold, and increased if the distance is smaller than the threshold. Likewise, the confidence factor of an imbalance estimation based on the output of a crankshaft torque sensor may be increased during cold start, and decreased during steady state operation. As a result, a cylinder-specific imbalance estimate collected during a drive cycle at less reliable conditions may be weighted less while a cylinder-specific imbalance estimate collected during the drive cycle at more reliable conditions may be weighted more. As such, the method allows shortcomings of any single estimation approach to be overcome, improving the overall accuracy and reliability of the cylinder imbalance estimation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.